Gas turbines are employed in many areas for driving generators or working machines. In this context, the energy content of a fuel is utilized for generating a rotational movement of a turbine shaft. The fuel is burnt in a number of burners for this purpose, air compressed by an air compressor being supplied. Owing to the combustion of the fuel, a working medium which is under high pressure and which has a high temperature is generated. This working medium is routed into a following turbine unit where it expands so as to perform work.
The turbine unit of a gas turbine has, for the pulse transmission of the working medium to the turbine shaft, a number of rotatable moving blades which are connected to the turbine shaft. For this purpose, the moving blades are arranged in a ring on the turbine shaft and thus form a number of moving blade rings or moving blade rows. The turbine and the compressor are arranged on a common turbine shaft, also designated as a turbine rotor, to which the generator or the working machine is also connected and which is mounted rotatably about its center axis.
Furthermore, the turbine unit usually comprises a number of stationary guide blades which are likewise fastened in a ring to an inner casing or the stator of the turbine so as to form guide blade rings or guide blade rows. The moving blades in this case serve for driving the turbine shaft by pulse transmission from the working medium flowing through the turbine. By contrast, the guide blades serve for routing the flow of the working medium in each case between two moving blade rows or moving blade rings which follow one another, as seen in the flow direction of the working medium. A successive pair consisting of a ring of guide blades or a guide blade row and of a ring of moving blades or a moving blade row is in this case also designated as a turbine stage.
A guide blade, as a rule, has a platform, designated as a blade root, which is arranged as a wall element for fixing the respective guide blade to the inner casing of the turbine and which forms the outer boundary of a hot-gas duct for the working medium flowing through the turbine. For an efficient flow routing of the working medium in the direction of the moving blade row following a guide blade row, a guide blade assigned to the guide blade row conventionally has a curved wing-shaped cross-sectional profile, so that the intended flow routing is established, with frictional losses on the respective guide blade being kept as low as possible, and therefore the guide blade row or the turbine stage assigned to it possesses as high efficiency as possible. For this purpose, the leading edge of a guide blade has a round cross section which narrows toward the tapering trailing edge of the guide blade. A moving blade is similarly shaped, as a rule specific details, such as, for example, the maximum profile thickness, the radius of curvature at the leading edge, etc., being adapted to the application, that is to say being optimized for a particularly efficient pulse transmission from the working medium to the respective moving blade.
In the design of gas turbines described above, particularly high efficiency is usually a design aim in addition to the achievable power. An increase in efficiency can in this case basically be achieved, for thermodynamic reasons, by an increase in the temperature at which the working medium flows out from the combustion chamber and into the turbine unit. Temperatures of about 1200° C. to 1500° C. for gas turbines of this type are therefore aimed at and even achieved.
With such high temperatures of the working medium, however, the components and structural parts exposed to this medium are exposed to high thermal loads. In order nevertheless to ensure a comparatively long service life of the relevant components with a high degree of reliability, it is normally necessary to cool the respective components, in particular the turbine blades. In order to prevent thermal distortions of the material, which limit the service life of the components, the aim, as a rule, is to achieve as uniform a cooling of the components as possible. The coolant used is in this case normally cooling air to which the heat of the structural parts to be cooled is transmitted. In this case, in what is known as impact cooling, the cooling air can be conducted essentially perpendicularly onto a surface to be cooled, or, in what is known as film cooling, it can be led along the surface to be cooled, that is to say essentially tangentially with respect to the latter. Furthermore, for the convective cooling of turbine components, cooling-air ducts integrated into these may be provided. Finally, the various cooling concepts may also be combined with one another.
To cool the turbine blades, in particular the guide blades, which are subjected to particularly high thermal load, cooling air is usually conducted into their interior, so that the walls of the respective turbine blade are cooled from inside. At least some of this cooling air is blown out of the blade interior through outlet ports from the trailing edge of the turbine blade rearwardly in the flow direction of the working medium. In order to configure the trailing edge in an aerodynamically beneficial way or to make it possible to have an effective cooling of the comparatively slender trailing edge by the blown-out cooling air, with the aerodynamically particularly beneficial contour being maintained on the suction side of the blade leaf the delivery-side blade wall originally converging in a wedge-shaped manner in the trailing edge region with the suction-side blade wall is cut back to an extent such that that portion of the suction-side blade wall which projects beyond the end edge of the delivery-side blade wall forms what is known as a trailing edge tab of small thickness. This trailing edge configuration is also designated as a cut-back trailing edge. Depending on production, other designations are also common, such as, for example, pressure side bleed. The trailing edge tab is cooled by means of film cooling by the cooling air flowing out from the gap between the end edge of the delivery-side blade wall and the trailing edge tab of the suction-side blade wall. The free-standing portions of the trailing edge tab are interrupted by stiffening portions or webs, also designated as land, in which, to stabilize the turbine blade in the trailing edge region, the delivery-side blade wall is extended in each case as far as the trailing edge end, so that the trailing edge is maintained with a comparatively high mass there.
The problem, here, is that, despite the described cooling of the trailing edge tab, type-induced overheating may occur in this region. In particular, crack formation caused thereby has the effect of limiting the service life of the respective turbine blade. Since, in the maintenance work necessary for eliminating such wear damage, mostly large parts of the gas turbine or of the respective turbine unit have to be demounted and subsequently reassembled, relatively long shutdown times also occur in addition to the costs for the procurement and installation of the replacement parts. Although the erosion of the turbine blades, particularly in the region of their respective trailing edge, can be counteracted by the increased use of cooling air, this nevertheless reduces the overall efficiency of the gas turbine.